The present invention relates to transmit diversity in wireless transmitters and communication systems. More particularly, the present invention relates to a method and apparatus for increasing transmit diversity efficiency using complex orthogonal space-time block codes.
Transmit diversity using multiple transmit antennas and, particularly, space-time block codes has recently attracted a remarkable attention in the communication literature and in the standardization bodies. Transmit diversity provides greatly improved performance on channels subject to multipath fading through the provision of a number of spatially separated replicas of the transmitted signal as may further be described in V. Tarokh, N. Seshadri and A. R. Calderbank, xe2x80x98Space-Time Codes for High Data Rate Wireless Communication: Performance Criterion and Code Construction, xe2x80x99IEEE Trans. on Information Theory, Vol. 44, No. 2, pp. 744-765, March 1998; S. Alamouti and V. Tarokh, xe2x80x98Transmitter diversity technique for wireless communicationsxe2x80x99, U.S. patent application, International Publication Number WO 99/14871, 25 Mar. 1999; and S. M. Alamouti, xe2x80x98A Simple Transmit Diversity Technique for Wireless Communicationsxe2x80x99, IEEE Journal on Selected Areas in Communications, Vol. 16, NO. 8, pp. 1451-1458, October 1998.
Transmit diversity involves replicas of the transmitted signal being communicated to a receiver on different and independent communication channels, each with a separate transmit antenna, thus increasing the probability that the receiver will receive at least some of the transmit signal replicas which have been less attenuated by fading and related multipath anomalies. Simultaneous signals transmitted from different transmit antennae in a transmit diversity environment have the same information content and differ only in the time domain. Such signal conditions are favorable for maximum likelihood decoding and allow maximum spatial diversity gains to be achieved provided some kind of space-time coding is applied to generate the simultaneous signals.
A simple form of space-time block coding is incorporated in a transmit antenna diversity scheme accepted for the third generation cellular standard (UTRA/FDD) as described in the 3rd Generation Partnership Program (3GPP)""s, Techn. Spec. TS 25.211, Physical channels and mapping of transport channels onto physical channels (FDD), September 1999. The transmit diversity scheme described therein, using two transmit antennas, is generally equivalent to another block coding scheme proposed, for example, by Alamouti as further described in xe2x80x9cTransmitter diversity technique for wireless communicationsxe2x80x9d, supra and xe2x80x9cA Simple Transmit Diversity Technique for Wireless Communicationsxe2x80x9d, supra. Compared to space-time trellis codes as described in xe2x80x98Space-Time Codes for High Data Rate Wireless Communication: Performance Criterion and Code Construction,xe2x80x9d supra, space-time block codes allow a much less complex decoding scheme to be used in the receiver. Despite certain performance losses compared to the space-time trellis codes, space-time block codes nevertheless provide the ability to achieve much lower decoding complexity making them a very attractive alternative for the practical applications and have merited further study as described in V. Tarokh, H. Jafarkhani and A. R. Calderbank, xe2x80x9cSpace-Time Block Codes from Orthogonal Designsxe2x80x9d, IEEE Trans. on Information Theory, Vol. 45, No. 5, pp. 1456-1467, July 1999.
Accordingly, lower decoding complexity transmit diversity may be achieved based on space-time block-codes having xe2x80x9cmxe2x80x9d information symbols which are coded into xe2x80x9cnxe2x80x9d codewords, each of length xe2x80x9cpxe2x80x9d code symbols. All codewords may then be transmitted simultaneously from xe2x80x9cnxe2x80x9d antennas. The minimum receiver complexity can be obtained if the codewords are orthogonal. At the same time, the codewords must be unique (and orthogonal) for any particular content of information symbols. Accordingly, space-time block codewords may preferably be obtained by weighting, conjugation and repetition of information symbols, or by combination of these three operations. The transmission rate R, defined as the ratio between the number of information symbols to the number of transmitted symbols in a codeword, is desirable to be as large as possible, ideally equal to 1. It should be noted that the receiver complexity is proportional to the length of codewords. For given number of antennas n, the minimum length of orthogonal codewords IS Pmin=n.
A larger transmit diversity gain may be obtained using a larger number of transmit antennas. Space-time block codes useful for larger numbers of transmit antennas are studied in xe2x80x9cSpace-Time Block Codes from Orthogonal Designsxe2x80x9d, supra, where a systematic complex space-time block coding method is developed, that for any number of antennas n greater than 2 codes of rate R=0.5, having the codeword""s length p greater than or equal to 2n may be produced. In addition to systematic code construction, two sporadic codes of rate R=0.75, for n=3 and n=4 are presented. It should be noted that the code for n=3 may actually be obtained from the code for n=4 by deleting a single codeword. The sporadic space-time code for n=4 has codewords of length p=4. However, information symbols in the codewords are weighted by the multilevel coefficients, so the actual transmitted signals from each of the antennas have non-constant envelopes regardless of whether the information symbols are associated with a constant-envelope modulation format (e.g. BPSK, MSK, etc.).
It should be noted that all the above codes may provide maximum spatial diversity order (gain) for a given number of transmit antennas. The conditions for maximum spatial diversity order is that each codeword contains all of information symbols, and that each information symbol is repeated an equal number of times within the codeword. It should further be noted that all the above codes may be valid for any arbitrary modulation format of information symbols. If the modulation format is restricted to be a constant-envelope modulation scheme, an alternative space-time block code of minimum length may be derived by using any Hadamard matrix as described in J.-C. Guey, M. P. Fitz, M. R. Bell and W.-Y. Kuo, xe2x80x98Signal design for transmitter diversity wireless communication systems over Rayleigh fading channels,xe2x80x99 in Proc. 1996 Veh. Technol. Conf., Atlanta, Ga., 1996, pp. 136-140. Namely, if each column of some xe2x80x9cnxc3x97nxe2x80x9d Hadamard matrix is multiplied symbol-by-symbol with the same string of n information symbols with constant envelope, the resulting matrix is the orthogonal space-time block code of minimum length and of rate R=1. It will be appreciated that the space-time block codewords are the columns of the resulting orthogonal matrix. It should further be noted that information symbols transmitted according to a constant-envelope modulation format all have a constant absolute value.
Problems arise however, in that systematic complex space-time codes proposed in xe2x80x9cSpace-Time Block Codes from Orthogonal Designsxe2x80x9d, supra may have twice or more times longer codeword""s length than the minimal possible (pmin=n). Hence they are not optimal from the decoder complexity point of view. Further problems arise in that a sporadic space-time block code for n=4, may have the minimum codeword length (p=4), however, information symbols are weighted by the multilevel coefficients leading to more difficult requirements for associated power amplifiers. Power amplifiers configured to deal with transmission based on sporadic space-time block codes, must deal with the more stringent requirements associated with higher peak power and higher range of linearity.
Still further problems arise in that space-time block codes described, for example, in xe2x80x98Signal design for transmitter diversity wireless communication systems over Rayleigh fading channelsxe2x80x9d, supra are both of a minimum length and of a maximum rate, however, they are valid only for constant-envelope modulation formats. Moreover, such codes are non-cyclic, leading to a disadvantage in that the use of cyclic codes generally lead to a decrease in decoder complexity. It should be noted that cyclic codes as will be understood by those skilled in the art are an important subclass of linear block codes which can easily be implemented in hardware using, for example, feedback shift registers and the like. A cyclic code may be defined in simple terms as a code where a subsequent code vector may be obtained from a previous code vector with one or more end around or cyclic shifts depending on, for example, the order of the underlying generator polynomial.
Thus, it can be seen that while the above mentioned methods address certain issues and problems, the difficulty posed by, for example, the use of many transmit antennae remains inadequately addressed. It would therefore be appreciated in the art for a method and apparatus for new cyclic complex space-time block codes having the minimum possible codeword length for a given number of antennas, in order to minimize decoder complexity increasing the efficiency of transmit diversity when larger numbers of transmit antennas are used. Information symbols in such space-time codes should be weighted by constant magnitude coefficients and should be valid for constant-envelope modulation formats or any arbitrary modulation formats of information symbols.
To reduce the impact of the use of larger numbers of antennas, a method and apparatus for transmitting a signal in a transmit diversity wireless communication system using n antennae in accordance with one embodiment of the present invention includes constructing an nxc3x97n orthogonal matrix Gn having n columns and n rows. Each column and row corresponds to n cyclic shifts of a modulatable orthogonal sequence gm, where m is a number of information symbols. The m information symbols may further be coded into n codewords corresponding to n columns of the matrix Gn. Different ones of the n codewords simultaneously transmitted on corresponding ones of the n transmit antennae. It should be noted that the n codewords may also correspond to the n rows of the matrix Gn.
In accordance with alternative exemplary embodiments, the n codewords may also correspond to the n rows of the matrix Gn obtained by permuting the n columns of the matrix Gn, to the n columns of the matrix Gn obtained by permuting the n rows of the matrix Gn, to a complex-conjugated version of the n codewords, to a sign-inverted version of the n codewords, or to a sign-inverted and complex-conjugated version of the n codewords.
In accordance with yet: another exemplary embodiment having 4 transmit antennae, a first row of a multilevel orthogonal 4xc3x97n matrix Gn(ML) may be configured as an arbitrary cyclic version of a GCL sequence of length n, a second row of a multilevel orthogonal matrix Gn(ML) as the complex-conjugated version of the first row having inverted signs of the first n/2 or the last n/2 out of consecutive symbols in the first row a third row as a reverted version of the first row, and a fourth row as a complex-conjugated version of the third row having inverted signs of a first n/2 or a last n/2 of consecutive symbols in the row. Thus codewords of a complex space-time block code may be taken as the rows of Gn(ML). For a GCL sequence of length 4, rows or columns of a multilevel orthogonal matrix G4(ML) may be taken as codewords.
In still another exemplary embodiment, a signal in a transmit diversity wireless communication system having n transmit antennae, may be decoded by receiving n signal samples and forming a received sequence of length n representing, for example, an unknown block of information symbols. The received sequence may be correlated with a reference sequence associated with the complex orthogonal space-time block code. The reference sequence may be cyclically shifted nxe2x88x921 times and each of the shifted reference sequences correlated with the received sequence. Moreover each of the n cyclic correlations generated may be weighted with a complex conjugate of an estimated channel coefficient and summed in order to obtain a decision metric. Additional decision metrics may further be generated for additional reference sequences associated with the complex orthogonal space-time block code. The decoded block of information symbols corresponding to the unknown block of information symbols represented by the received signal samples may be selected as corresponding to the reference sequence which produces a decision metric which is favorable.